The automatic direction finder (ADF), long found on most civil aircraft panels, doesnít aid navigation in an entirely straightforward way.
Although the ADF needle points directly at low-frequency non-directional beacons (NDB), it doesnít indicate a heading to the station. To get the magnetic bearing to the station, the ADFs fixed compass card requires the pilot to take the relative bearing to or from the station (the angle between the aircraftís nose or tail and the NDB), and then add the relative bearing to the current magnetic heading to get the magnetic bearing to the station.
An ADF with a rotatable card partially solves this problem by allowing the pilot to rotate the card to match the heading on the heading indicator. While this provides a magnetic heading to the station, the pilot must reset the heading indicator to the magnetic compass in level flight every ten minutes or so, adding an additional manual task.
The radio magnetic indicator (RMI) is one solution to the ADF's shortcomings. The RMI combines three components: a fluxgate, a heading indicator, and a relative bearing indicator.
Some RMIs have two needles like the one pictured on this page; others only have one needle. The RMI can be used for VOR navigation as well as ADF navigation. Most single-needle RMIs have a switch that allows the pilot to select either an ADF or VOR station to which the needle can point.
In Flight Simulator, all of the RMIs are dual-needle and only provide information for the ADF and VOR 2. You cannot select a different radio for each needle. The ADF needle is yellow and the VOR needle is green. The VOR needle points to a VOR station and the tail of the needle indicates the current radial.
Green needle: VOR
Yellow needle: ADF
The fluxgate is a detector that is sensitive to magnetic north. It is mounted in a relatively non-magnetic place in the airplane to minimize magnetic interference. The fluxgate constantly and automatically updates the RMIís heading indicator, eliminating the task of turning a compass card or correcting the RMIís heading indicator to the magnetic compass.
With the magnetic bearing indicator pointing at an NDB, the current heading to or from the station is readily apparent. The ADF needle indicates the magnetic heading to the station and the tail of the pointer indicates the magnetic heading away from the station. Depending on whether the pilot wants to fly to or from the station, he just turns the aircraft to the heading indicated by the ADF pointer.
NDB approaches and RMI
An RMI can simplify flying NDB approaches by eliminating the need to add magnetic heading calculations into the IFR task load. The aircraftís position relative to the station is always clear, whether flying to or from the station.
In the example here, the pilot is flying outbound for a course reversal. With the tail of the ADF needle centered at the top of the RMI, the magnetic heading is the reciprocal of the final approach heading. The final approach course is 157 degrees, the outbound heading is 337 degrees. A right-hand procedure turn will make the heading 022 degrees and the tail of the RMI needle will point to 337 degrees.
Upon reversal of the procedure turn, the heading is 202 and the head of the RMI needle points to 157. As the needle moves closer to the left wingtip the pilot turns the aircraft onto the final approach course, centering the RMI needle on the final approach heading of 157. Remember that the pointer always points at the station and the RMI compass card displays the current magnetic heading. If the needle points ahead of the wing the station is ahead of the aircraft. Similarly, if the needle points behind the wing the station is behind the aircraft.
RMI and DME Arcs
Flying a DME arc can also benefit from reference to the RMI. Until reaching the radial that represents the final approach course, the pilot flies the arc by keeping the aircraft a specified distance away from a VOR/DME station.
In the example pictured here, there is an arc seven nautical miles (7 DME) from a VOR and the final approach course is 109 degrees. The pilot flies the arc by keeping the RMI needle pointed towards the left wingtip while flying a series of short straight legs.
A DME arc provides an 8-mile wide corridor, but the goal should be to stay within one nautical mile of the arc.
As the pilot flies a straight line tangential to the arc, the DME distance begins to increase and the needle moves behind the wingtip. When the needle is 10 degrees behind the wingtip, the pilot turns 20 degrees in the direction of the arc, which moves the RMI needle 10 degrees ahead of the wingtip. The pilot holds that heading until the needle falls behind the wingtip again, and then repeats this procedure until it is time to turn inbound on the final approach course. In all of the examples, a no-wind situation is assumed.
Simple and easy-to-use, itís little wonder why the RMI is on the instrument panel of many complex aircraft today.
Flight Simulator aircraft with RMI:
- Beechcraft Baron
- Beechcraft King Air
- Boeing 747-400